This application claims the benefit of the filing date of Taiwan Application Ser. No 095110189, filed on Mar. 24 2006, the content of which is incorporated herein by reference.
1. Field of Invention
The invention relates to an operational amplifier driver and a method of canceling an offset voltage of an operational amplifier.
2. Related Art
An output voltage of a source driver from a typical thin film transistor liquid crystal display (TFT-LCD) is driven by an operational amplifier. Thus, an offset voltage of the operational amplifier influences the output voltage of the source driver. The offset voltage of the operational amplifier may vary due to the variation of the manufacturing processes so as to influence the color quality of the LCD. Thus, the problem of canceling the offset voltage has to be solved in the source driver.
FIG. 1A shows a conventional operational amplifier driver for canceling an offset voltage. Referring to FIG. 1A, the operational amplifier driver 10 includes three switches 11, 12 and 13, one capacitor 14 and one operational amplifier 15. An input signal Vin is inputted to the positive input terminal of the operational amplifier 15 through the switch 11. The input signal Vin is additionally inputted to the negative input terminal of the operational amplifier 15 through the switches 12 and 13. The capacitor 14 is connected to the positive input terminal of the operational amplifier 15 and a contact point between the switches 12 and 13. In addition, an output terminal of the operational amplifier 15 is connected to the negative input terminal for feedback. The operational amplifier driver 10 operates in two stages including an offset voltage sampling stage and an offset voltage holding stage.
FIG. 1B shows a switching state in the offset voltage sampling stage of the operational amplifier driver of FIG. 1A. FIG. 1C shows a switching state in the offset voltage holding stage of the operational amplifier driver of FIG. 1A. As shown in FIG. 1B, the switches 11 and 13 are turned on and the switch 12 is turned off in the offset voltage sampling stage. Thus, the offset voltage of the operational amplifier 15 is stored in the capacitor 14 across the two terminals of the capacitor 14. As shown in FIG. 1C, the switches 11 and 13 are turned off and the switch 12 is turned on in the offset voltage holding stage. Thus, the voltage stored in the capacitor 14 across the two terminals of the capacitor 14 is offset against the offset voltage of the operational amplifier 15 such that an output terminal voltage Vout of the operational amplifier 15 is equal to an input voltage Vin.
However, the above-mentioned technology needs the input voltage to directly charge the capacitor, so the level of the input signal has to be high enough. When the level of the input signal is not high enough, the effect of the above-mentioned technology is not ideal and thus influences the response speed.